Fan

ABSTRACT

A fan includes a motor and an impeller. The motor includes a stationary portion and a rotating portion. The stationary portion includes a stator and a bearing portion. The bearing portion includes a sleeve and a bearing housing. The rotating portion includes a rotor magnet, a shaft, and a thrust plate. A radial gap includes a radial dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting axially downward on a lubricating oil, while a thrust gap includes a thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting radially inward on the lubricating oil. A circulation hole extending in an axial direction is defined between the bearing housing and the sleeve. The radial gap is arranged to have a radial width in a range of about 5 μm to about 20 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fan arranged to produce air currents.

2. Description of the Related Art

Cooling fans arranged to cool electronic components have been installed inside cases of a variety of electronic devices. A motor portion of an axial fan includes a base portion, an armature, a substantially cylindrical bearing support portion, two ball bearings, and a rotor portion. The bearing support portion is fixed in a center of the base portion. The two ball bearings are fixed to an inside surface of the bearing support portion, while the armature is fixed to an outside surface of the bearing support portion. A shaft is inserted in the ball bearings, so that the rotor portion is supported to be rotatable with respect to the bearing support portion. The base portion includes an annular groove defined therein which is arranged to surround a circumference of the bearing support portion. A helical coil spring is disposed in this groove. An upper end portion of the coil spring is arranged to be in axial contact with an insulator of the armature. Thus, vibrations of the armature are absorbed by the coil spring during rotation of the rotor portion, so that a reduction in vibrations of the axial fan is achieved.

A bearing apparatus used in a spindle motor includes a shaft, a thrust plate, a sleeve, and a housing arranged in the shape of a cylinder with a bottom. The shaft is inserted in the sleeve. The housing is arranged to accommodate the sleeve. The thrust plate is arranged at a lower end portion of the shaft. An inner circumferential surface of the sleeve includes dynamic pressure generating grooves defined therein, and a radial dynamic pressure bearing is defined between an outer circumferential surface of the shaft and the inner circumferential surface of the sleeve. Each of a lower end surface of the sleeve and an upper surface of a bottom portion of the housing includes thrust dynamic pressure generating grooves defined therein. Thrust dynamic pressure bearings are defined between the lower end surface of the sleeve and an upper surface of the thrust plate, and between a lower surface of the thrust plate and the upper surface of the bottom portion of the housing.

In recent years, electronic devices, such as servers, have improved in performance, and the amount of heat generated from the electronic devices has increased accordingly. There is therefore a demand for cooling fans in the electronic devices to be rotated at higher speeds in order to increase air volume. However, an increase in the rotation speed of the cooling fans leads to greater vibrations of the cooling fans, and this will affect other devices in the electronic devices. For example, a vibration of a cooling fan may cause an error in reading or writing by a disk drive apparatus.

SUMMARY OF THE INVENTION

A fan according to a preferred embodiment of the present invention includes a motor and an impeller including a plurality of blades, and arranged to rotate about a central axis through the motor to produce air currents. The motor includes a stationary portion and a rotating portion rotatably supported by the stationary portion. The stationary portion includes a stator and a bearing portion arranged inside of the stator. The bearing portion includes a sleeve defined by a metallic sintered body, and a bearing housing arranged to cover an outer circumferential surface of the sleeve. The rotating portion includes a rotor magnet arranged radially outside the stator; a shaft inserted in the sleeve, and having an upper portion fixed to the impeller directly or through one or more members; and a thrust plate arranged to extend radially outward from a lower end of the shaft to be axially opposed to a lower surface of the sleeve. A radial gap defined between an inner circumferential surface of the sleeve and an outer circumferential surface of the shaft includes a radial dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting axially downward on a lubricating oil, while a thrust gap defined between the lower surface of the sleeve and an upper surface of the thrust plate includes a thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting radially inward on the lubricating oil. A circulation hole extending in an axial direction from an upper surface to the lower surface of the sleeve is defined between the bearing housing and the outer circumferential surface of the sleeve. The radial gap is arranged to have a radial width in a range of about 5 μm to about 20 μm.

A fan according to another preferred embodiment of the present invention includes a motor and an impeller including a plurality of blades, and arranged to rotate about a central axis through the motor to produce air currents. The motor includes a stationary portion and a rotating portion rotatably supported by the stationary portion. The stationary portion includes a stator and a bearing portion arranged inside of the stator. The bearing portion includes a sleeve defined by a metallic sintered body, and a bearing housing arranged to cover an outer circumferential surface of the sleeve. The rotating portion includes a rotor magnet arranged radially outside the stator; a shaft inserted in the sleeve, and having an upper portion fixed to the impeller directly or through one or more members; and a thrust plate arranged to extend radially outward from a lower end of the shaft to be axially opposed to a lower surface of the sleeve. A radial gap defined between an inner circumferential surface of the sleeve and an outer circumferential surface of the shaft includes a radial dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting on a lubricating oil, while a thrust gap defined between the lower surface of the sleeve and an upper surface of the thrust plate includes a thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting on the lubricating oil. A circulation hole extending in an axial direction from an upper surface to the lower surface of the sleeve is defined between the bearing housing and the outer circumferential surface of the sleeve. The lubricating oil is arranged to flow from the radial gap back to the radial gap through a gap above the upper surface of the sleeve, the circulation hole, and the thrust gap in an order named during rotation of the rotating portion.

The fans according to the preferred embodiments of the present invention are able to achieve reduced vibration.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a fan according to a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view of a bearing mechanism according to a preferred embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a portion of the bearing mechanism in an enlarged form.

FIG. 4 is a cross-sectional view of a bearing portion according to a preferred embodiment of the present invention.

FIG. 5 is a bottom view of the bearing portion.

FIG. 6 is a plan view of a thrust cap according to a preferred embodiment of the present invention.

FIG. 7 is a cross-sectional view illustrating a portion of the bearing mechanism in an enlarged form.

FIG. 8 is a graph showing a result of a simulation of vibration that occurs in the fan.

FIG. 9 is a graph showing a result of a simulation of vibration that occurs in the fan.

FIG. 10 is a graph showing a result of a simulation of vibration that occurs in the fan.

FIG. 11 is a graph showing a result of a simulation of vibration that occurs in the fan.

FIG. 12 is a graph showing a result of a simulation of vibration that occurs in a fan as a comparative example.

FIG. 13 is a graph showing a relationship between the flow of a lubricating oil which circulates during drive of a motor and the radial width of a radial gap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is assumed herein that a vertical direction is defined as a direction in which a central axis of a motor extends, and that an upper side and a lower side along the central axis in FIG. 1 are referred to simply as an upper side and a lower side, respectively. It should be noted, however, that the above definitions of the vertical direction and the upper and lower sides should not be construed to restrict relative positions or directions of different members or portions when the motor is actually installed in a device. Also note that a direction parallel to the central axis is referred to by the term “axial direction”, “axial”, or “axially”, that radial directions centered on the central axis are simply referred to by the term “radial direction”, “radial”, or “radially”, and that a circumferential direction about the central axis is simply referred to by the term “circumferential direction”, “circumferential”, or “circumferentially”.

FIG. 1 is a cross-sectional view of an axial fan 1 according to a preferred embodiment of the present invention. Hereinafter, the axial fan 1 will be referred to simply as the “fan 1”. The fan 1 includes a motor 11, an impeller 12, a housing 13, a plurality of support ribs 14, and a base portion 15. The housing 13 is arranged to surround an outer circumference of the impeller 12. The housing 13 is joined to the base portion 15 through the support ribs 14. The support ribs 14 are arranged in a circumferential direction. The base portion 15 is defined integrally with the support ribs 14. The motor 11 is fixed on the base portion 15.

The impeller 12 is made of a resin, and includes a cup 121 and a plurality of blades 122. The cup 121 is arranged substantially in the shape of a covered cylinder. The cup 121 is arranged to cover an outside of the motor 11. The cup 121 is arranged to define a portion of a rotating portion 2 of the motor 11. The rotating portion 2 will be described below. The cup 121 includes a top face portion 123 and a side wall portion 124. The top face portion 123 is arranged to spread perpendicularly to a central axis J1. The side wall portion 124 is arranged to extend downward from an outer edge portion of the top face portion 123. The blades 122 are arranged to extend radially outward from an outer circumferential surface of the side wall portion 124 with the central axis J1 as a center. The cup 121 and the blades 122 are defined integrally with each other by a resin injection molding process.

A hole portion 125 is defined in an upper surface of the top face portion 123. A weight 129 is arranged in the hole portion 125. The weight 129 is an adhesive including a metal having a high specific gravity, such as tungsten. Another weight 129 is arranged on a lower end portion 124 a of the side wall portion 124 on a radially inner side thereof. A reduction in unbalance of each of the impeller 12 and the rotating portion 2 of the motor 11 can be achieved by arranging the weight 129 on each of an upper portion and a lower portion of the impeller 12. Two-plane balance correction as described above achieves a reduction in vibrations of the fan 1 owing to a displacement of a center of gravity of any of the impeller 12 and the motor 11 from the central axis J1. Hereinafter, the hole portion 125 and the lower end portion 124 a of the side wall portion 124, on each of which the weight 129 is arranged, will be referred to as “balance correction portions 125 and 124 a”, respectively.

The impeller 12 of the fan 1 is caused by the motor 11 to rotate about the central axis J1 to produce downward air currents.

The motor 11 is a three-phase outer-rotor motor. The motor 11 includes the rotating portion 2, a stationary portion 3, and a bearing mechanism 4. As described below, the bearing mechanism 4 can be considered to be defined by a portion of the rotating portion 2 and a portion of the stationary portion 3. The rotating portion 2 includes a substantially cylindrical metallic yoke 21, a rotor magnet 22, and the cup 121. The yoke 21 is fixed to an inside of the cup 121. The rotor magnet 22 is fixed to an inner circumferential surface of the yoke 21. The rotor magnet 22 is arranged radially outside a stator 32, which will be described below. The rotating portion 2 is supported through the bearing mechanism 4 to be rotatable about the central axis J1 with respect to the stationary portion 3.

The stationary portion 3 includes a substantially cylindrical bearing support portion 31, the stator 32, and a circuit board 33. A lower portion of the bearing support portion 31 is fixed to an inner circumferential surface of the base portion 15 which defines a central hole portion thereof. The stator 32 is fixed to an outer circumferential surface of the bearing support portion 31 on an upper side of the base portion 15. The stator 32 includes a stator core 321 and a plurality of coils 322 arranged on the stator core 321. The stator core 321 is defined by laminated steel sheets. The circuit board 33 is fixed below the stator 32. Lead wires from the coils 322 are attached to pins (not shown) inserted in holes of the circuit board 33, whereby the stator 32 and the circuit board 33 are electrically connected with each other. Note that the lead wires from the coils 322 may be directly connected to the circuit board 33. During drive of the motor 11, a turning force is generated between the rotor magnet 22 and the stator 32.

An annular magnetic member 331 is arranged on an upper surface of the circuit board 33. The magnetic member 331 is arranged under the rotor magnet 22. While the motor 11 is stationary, a magnetic center of the stator 32 is located at a level lower than that of a magnetic center of the rotor magnet 22. In the fan 1, magnetic attraction forces that attract the rotor magnet 22 downward are generated between the rotor magnet 22 and the stator 32, and between the rotor magnet 22 and the magnetic member 331. A force that acts to lift the impeller 12 relative to the stationary portion 3 during rotation of the fan 1 is thereby reduced.

FIG. 2 is a cross-sectional view illustrating the bearing mechanism 4 and its vicinity in an enlarged form. The bearing mechanism 4 includes a shaft 41, an annular thrust plate 42, a bearing portion 44, a thrust cap 45, i.e., a bearing bottom portion, and a lubricating oil 46. Referring to FIG. 1, an upper portion of the shaft 41 is indirectly fixed to the top face portion 123 of the impeller 12 through a bushing 25 made of a metal. Referring to FIG. 2, the thrust plate 42 is fixed to a lower portion of the shaft 41. The thrust plate 42 is arranged to extend radially outward from a lower end of the shaft 41. The thrust plate 42 is arranged axially opposite a lower surface 472 of a sleeve 47, which will be described below, of the bearing portion 44. An upper surface of the thrust plate 42 includes a substantially annular surface 422 arranged to extend perpendicularly to the central axis J1 around the shaft 41. Hereinafter, this surface 422 will be referred to as an “upper annular surface 422”. A lower surface of the thrust plate 42 includes a substantially annular surface 423 arranged to extend perpendicularly to the central axis J1. Hereinafter, this surface 423 will be referred to as a “lower annular surface 423”. The bearing portion 44 is arranged radially inward of the stator 32. Note that each of the shaft 41 and the thrust plate 42 defines a portion of the rotating portion 2 illustrated in FIG. 1, while each of the bearing portion 44 and the thrust cap 45 defines a portion of the stationary portion 3.

The bearing portion 44 illustrated in FIG. 2 includes the sleeve 47 and a tubular bearing housing 48. The sleeve 47 is tubular in shape, and is defined by a metallic sintered body. The sleeve 47 is impregnated with the lubricating oil 46. The bearing housing 48 is arranged to cover an outer circumferential surface 473 of the sleeve 47. The bearing housing 48 includes an annular upper portion 481, which is substantially annular in shape and is arranged to extend radially inward on an upper side of the sleeve 47. The bearing housing 48 is fixed to an inner circumferential surface of the bearing support portion 31. Below the annular upper portion 481, an inner circumferential surface 482 of the bearing housing 48 is arranged radially opposite the outer circumferential surface 473 of the sleeve 47.

The outer circumferential surface 473 of the sleeve 47 includes a plurality of grooves extending in an axial direction defined therein. Each of the grooves is arranged to extend from an upper surface 474 to the lower surface 472 of the sleeve 47. Each of the grooves becomes one of a plurality of circulation holes 445 as a result of contact of the inner circumferential surface 482 of the bearing housing 48 with the outer circumferential surface 473 of the sleeve 47. That is, the inner circumferential surface 482 of the bearing housing 48 and the surface which defines the plurality of grooves together define the plurality of circulation holes 445. Each of the circulation holes 445 is defined between the outer circumferential surface 473 of the sleeve 47 and the inner circumferential surface 482 of the bearing housing 48. Each circulation hole 445 is arranged to extend in the axial direction from the upper surface 474 to the lower surface 472 of the sleeve 47. The circulation holes 445 are arranged at substantially regular intervals in the circumferential direction.

The lower surface 472 of the sleeve 47 is arranged in a substantially annular shape centered on the central axis J1. The shaft 41 is inserted in the sleeve 47. The sleeve 47 is arranged to have an axial length of about 13.0 mm in the present preferred embodiment. The sleeve 47 is arranged to have an inside diameter of about 3.0 mm and an outside diameter of about 6.7 mm. The thrust plate 42 is arranged to have an outside diameter of about 6.6 mm.

The thrust cap 45 is arranged inside of a lower end portion of the bearing housing 48. An outer circumferential surface of the thrust cap 45 is fixed to a lower end portion of the inner circumferential surface 482 of the bearing housing 48. The thrust cap 45 is arranged to close a bottom portion of the bearing housing 48. That is, the thrust cap 45 is a bottom portion of the bearing mechanism 4. The thrust cap 45 is arranged axially opposite the lower annular surface 423 of the thrust plate 42.

In the bearing mechanism 4, a radial gap 51 is defined between an inner circumferential surface 471 of the sleeve 47 and an outer circumferential surface 411 of the shaft 41. The upper annular surface 422 of the thrust plate 42 and the lower surface 472 of the sleeve 47 are arranged axially opposite each other. A gap 52 is defined between the upper annular surface 422 and the lower surface 472. Hereinafter, the gap 52 will be referred to as a “first lower thrust gap 52”. The lower annular surface 423 of the thrust plate 42 and an upper surface 451 of the thrust cap 45 are arranged axially opposite each other. A gap 53 is defined between the lower annular surface 423 and the upper surface 451. Hereinafter, the gap 53 will be referred to as a “second lower thrust gap 53”. The sum of the axial width of the first lower thrust gap 52 and the axial width of the second lower thrust gap 53 is arranged in the range of about 10 μm to about 40 μm. A gap 54 is defined between an outer circumferential surface of the thrust plate 42 and an inner circumferential surface of the thrust cap 45. Hereinafter, the gap 54 will be referred to as a “side gap 54”.

FIG. 3 is a diagram illustrating an upper portion of the bearing portion 44 and its vicinity in an enlarged form. An inner circumferential surface 481 a of the annular upper portion 481 of the bearing housing 48 is an inclined surface whose diameter gradually increases with increasing height. In other words, the inner circumferential surface 481 a is arranged to be inclined radially inward with decreasing height. Hereinafter, the inner circumferential surface 481 a will be referred to as a “first inclined surface 481 a”. An upper portion of the inner circumferential surface 471 of the sleeve 47 includes an inclined surface 471 a whose diameter gradually increases with increasing height. In other words, the inclined surface 471 a is arranged to be inclined radially inward with decreasing height. Hereinafter, the inclined surface 471 a will be referred to as a “second inclined surface 471 a”. An angle defined by the first inclined surface 481 a with the central axis J1 is arranged to be greater than an angle defined by the second inclined surface 471 a with the central axis J1.

A single seal gap 55 arranged to gradually increase in radial width with increasing height is defined between the first inclined surface 481 a and the outer circumferential surface 411 of the shaft 41. The seal gap 55 is arranged in an annular shape centered on the central axis J1. Below and adjacent to the seal gap 55, a gap 56 is defined between the outer circumferential surface 411 of the shaft 41 and the second inclined surface 471 a. The seal gap 55 includes a seal portion 55 a arranged to retain the lubricating oil 46 through capillary action. Because the seal portion 55 a is defined around the shaft 41, a leakage of the lubricating oil 46 out of the seal portion 55 a due to a centrifugal force is prevented. Moreover, the seal gap 55 serves as an oil buffer arranged to hold a large amount of the lubricating oil 46.

Referring to FIG. 2, in the motor 11, the seal gap 55, the radial gap 51, the first lower thrust gap 52, the side gap 54, and the second lower thrust gap 53 are arranged to together define a single continuous bladder structure 5, and the lubricating oil 46 is arranged continuously in the bladder structure 5. Within the bladder structure 5, a surface of the lubricating oil 46 is defined only in the seal gap 55.

Referring to FIG. 3, an upper surface of the annular upper portion 481 of the bearing portion 44 and a lower surface of the bushing 25, which is fixed to the upper portion of the shaft 41, are arranged to together define a gap 501 extending radially therebetween. An outer circumferential surface of the bushing 25 and the inner circumferential surface of the bearing support portion 31 are arranged to together define a gap 502 extending in the axial direction therebetween. The seal portion 55 a is arranged to be in communication with an exterior space through the gaps 501 and 502. Here, the exterior space refers to a space above the stator 32 as illustrated in FIG. 1. Provision of the gaps 501 and 502 contributes to preventing an air including a lubricating oil evaporated from the seal portion 55 a from traveling out of the bearing mechanism 4. This contributes to reducing evaporation of the lubricating oil 46 out of the bearing mechanism 4.

FIG. 4 is a vertical cross-sectional view of the sleeve 47. In FIG. 4, the inner circumferential surface 471 of the sleeve 47 is additionally depicted. An upper portion and a lower portion of the inner circumferential surface 471 of the sleeve 47 include a first radial dynamic pressure groove array 711 and a second radial dynamic pressure groove array 712, respectively, defined therein. Each of the first and second radial dynamic pressure groove arrays 711 and 712 is arranged in a herringbone pattern. The herringbone pattern of the first radial dynamic pressure groove array 711 is asymmetrical in the vertical direction. In an upper portion of the radial gap 51 illustrated in FIG. 2, an upper radial dynamic pressure bearing portion 681 arranged to generate a fluid dynamic pressure acting axially downward on the lubricating oil 46 is defined through the first radial dynamic pressure groove array 711. Meanwhile, the herringbone pattern of the second radial dynamic pressure groove array 712 is symmetrical in the vertical direction. In a lower portion of the radial gap 51, a lower radial dynamic pressure bearing portion 682 is defined through the second radial dynamic pressure groove array 712. Hereinafter, the upper and lower radial dynamic pressure bearing portions 681 and 682 will be referred to collectively as a “radial dynamic pressure bearing portion 68”.

The fluid dynamic pressure generated by the upper radial dynamic pressure bearing portion 681 located in the upper portion of the radial gap 51 is greater than a fluid dynamic pressure generated by the lower radial dynamic pressure bearing portion 682 located in the lower portion of the radial gap 51. Accordingly, the radial dynamic pressure bearing portion 68 as a whole generates a fluid dynamic pressure acting axially downward on the lubricating oil 46. The radial dynamic pressure bearing portion 68 is arranged axially between the two balance correction portions 124 a and 125 illustrated in FIG. 1. In addition, the upper radial dynamic pressure bearing portion 681 is arranged to overlap with a center of gravity of a combination of the motor 11 and the impeller 12 in a radial direction.

FIG. 5 is a bottom view of the sleeve 47. The lower surface 472 of the sleeve 47 includes a first thrust dynamic pressure groove array 721 arranged in a herringbone pattern. In the first lower thrust gap 52 illustrated in FIG. 2, a first lower thrust dynamic pressure bearing portion 691 arranged to generate a fluid dynamic pressure acting in a thrust direction (i.e., the axial direction) on the lubricating oil 46 is defined through the first thrust dynamic pressure groove array 721. Referring to FIG. 5, the first thrust dynamic pressure groove array 721 is made up of a plurality of first thrust dynamic pressure grooves 721 a. Each first thrust dynamic pressure groove 721 a includes a circumferential top 721 b, a portion 721 c on a radially outer side of the top 721 b, and a portion 721 d on a radially inner side of the top 721 b. The portion 721 c is arranged to have a radial width greater than that of the portion 721 d. The first lower thrust dynamic pressure bearing portion 691 illustrated in FIG. 2 is thus arranged to generate a fluid dynamic pressure acting radially inward on the lubricating oil 46. Note that the first thrust dynamic pressure groove array 721 is not limited to the herringbone pattern, but may be arranged in a spiral-in pattern. Even in this case, a fluid dynamic pressure acting radially inward on the lubricating oil 46 is generated by the first lower thrust dynamic pressure bearing portion 691.

FIG. 6 is a plan view of the thrust cap 45. Referring to FIG. 6, the upper surface 451 of the thrust cap 45, i.e., a bottom surface of the bladder structure 5 illustrated in FIG. 2, includes a second thrust dynamic pressure groove array 722 arranged in a herringbone pattern. In the second lower thrust gap 53, a second lower thrust dynamic pressure bearing portion 692 arranged to generate a fluid dynamic pressure acting in the thrust direction (i.e., the axial direction) on the lubricating oil 46 is defined through the second thrust dynamic pressure groove array 722. Note that the second thrust dynamic pressure groove array 722 is not limited to the herringbone pattern, but may be arranged in a spiral-in pattern.

During the drive of the motor 11, the shaft 41 is supported in the radial direction by the radial dynamic pressure bearing portion 68, while the thrust plate 42, which is arranged above a bottom portion of the bladder structure 5, is supported in the thrust direction by the first and second lower thrust dynamic pressure bearing portions 691 and 692. As a result, both the rotating portion 2 and the impeller 12 illustrated in FIG. 1 are supported to be rotatable with respect to the stationary portion 3.

During the drive of the motor 11, the lubricating oil 46 circulates through the radial gap 51, a gap 446 defined between a lower surface of the annular upper portion 481 and the upper surface 474 of the sleeve 47, the circulation holes 445, and the first lower thrust gap 52 illustrated in FIG. 2. The gap 446 defined between the lower surface of the annular upper portion 481 and the upper surface 474 of the sleeve 47 is, in other words, a gap above the upper surface 474 of the sleeve 47, and will therefore be hereinafter referred to as an “above-sleeve gap 446”. In the present preferred embodiment, the upper surface 474 of the sleeve 47 includes a plurality of grooves arranged to extend radially from the inner circumferential surface 471 to the outer circumferential surface 473 of the sleeve 47. Radially outer end portions of these grooves are joined to upper end portions of the circulation holes 445. The grooves define the above-sleeve gap 446 as a result of contact of the lower surface of the annular upper portion 481 with the upper surface 474 of the sleeve 47.

Referring to FIG. 4, a portion of the first radial dynamic pressure groove array 711 is defined in a lower portion of the second inclined surface 471 a. Referring to FIG. 3, if the shaft 41 is slightly tilted during drive of the fan 1, a fluid dynamic pressure is generated by the first radial dynamic pressure groove array 711 in the gap 56 between a portion of the outer circumferential surface 411 of the shaft 41 which approaches the second inclined surface 471 a and a portion of the second inclined surface 471 a which is opposed to this portion of the outer circumferential surface 411. As a result, the shaft 41 is supported by the second inclined surface 471 a. Thus, when the shaft 41 is tilted during rotation of the rotating portion 2, the second inclined surface 471 a extends along the outer circumferential surface 411 of the shaft 41 in the gap 56, which is located below and adjacent to the seal gap 55. The shaft 41 is thus prevented from coming into hard contact with the upper portion of the bearing portion 44.

FIG. 7 is a cross-sectional view illustrating the thrust plate 42 and its vicinity in an enlarged form. The upper surface of the thrust plate 42 includes an inclined surface 422 a defined in an outer edge portion thereof. The inclined surface 422 a is arranged radially outward of and adjacent to the upper annular surface 422. The inclined surface 422 a is arranged to be inclined downward with increasing distance from the central axis J1. The inclined surface 422 a is arranged axially opposite an outer edge portion of the lower surface 472 of the sleeve 47. The lower surface of the thrust plate 42 includes an inclined surface 423 a defined in an outer edge portion thereof. The inclined surface 423 a is arranged radially outward of and adjacent to the lower annular surface 423. The inclined surface 423 a is arranged to be inclined upward with increasing distance from the central axis J1. Provision of the inclined surface 422 a in the outer edge portion of the upper surface of the thrust plate 42 contributes to preventing the thrust plate 42 from coming into hard contact with the lower surface 472 of the sleeve 47 when the shaft 41 is tilted during the rotation of the rotating portion 2.

FIG. 8 is a graph showing a result of a simulation of vibration that occurs in the fan 1 in the case where the radial width of the radial gap 51 is 3 μm. A horizontal axis represents frequencies of the vibration, while a vertical axis represents the amplitude of each frequency component of the vibration. FIGS. 9, 10, and 11 are graphs showing results of simulations of vibration that occurs in the fan 1 in the case where the radial width of the radial gap 51 is 4 μm, 5 μm, and 6 μm, respectively. FIG. 12 is a graph showing a result of a simulation of vibration that occurs in a fan as a comparative example in which a motor including a ball bearing is installed.

As indicated by a curve 90 in FIG. 12, in the case of the vibration that occurs in the fan including the ball bearing, a plurality of peaks occur in the range of 750 Hz to 1250 Hz. In FIG. 12, the peaks are denoted, from right to left, by reference numerals 901, 902, 903, and 904, respectively. In contrast, referring to FIGS. 8 and 9, in the case of the bearing mechanisms 4 in which the width of the radial gap is 3 μm and 4 μm, respectively, corresponding peaks 911, 912, 913, and 914 are lower than the peaks 901, 902, 903, and 904, respectively, in FIG. 12. Further, referring to FIGS. 10 and 11, in the case of the bearing mechanisms 4 in which the width of the radial gap 51 is 5 μm and 6 μm, respectively, peaks do not occur at positions corresponding to those of the peaks 901 and 904 on the far right and on the far left, respectively, in FIG. 12. Moreover, peaks 912 and 913 corresponding to the remaining peaks 902 and 903, respectively, are less than half as high as the peaks 902 and 903, respectively.

As described above, because of use of the bearing mechanism 4, which is a fluid dynamic bearing mechanism, the fan 1 is able to achieve reduced vibration as compared to known fans in which ball bearings are used, due to a so-called damper effect produced by the lubricating oil 46 between the shaft 41 and the bearing portion 44. This leads to a reduction in power consumption of the fan 1. In particular, a satisfying reduction in the vibration can be achieved when the radial width of the radial gap 51 is 5 μm or greater. The radial width of the radial gap 51 is arranged to be less than about 20 μm in order to generate a sufficient fluid dynamic pressure in the radial gap 51. Preferably, the radial width of the radial gap 51 is arranged in the range of about 5 μm to about 10 μm.

FIG. 13 is a graph showing a relationship between the flow of the lubricating oil 46 which circulates during the drive of the motor 11 and the radial width of the radial gap 51. In FIG. 13, a horizontal axis represents the radial width of the radial gap 51, while a vertical axis represents the flow of the lubricating oil 46 in the radial gap 51. In FIG. 13, the upward flow of the lubricating oil 46 in the radial gap 51 is represented by positive values, while the downward flow of the lubricating oil 46 in the radial gap 51 is represented by negative values. As is shown in FIG. 13, the lubricating oil 46 flows upward in the radial gap 51 when the radial width of the radial gap 51 is greater than 5 μm.

When the radial width of the radial gap 51 is greater than 5 μm, the lubricating oil 46 flows upward in the radial gap 51, and from the radial gap 51 the lubricating oil 46 flows back to the radial gap 51 through the above-sleeve gap 446, the circulation holes 445, and the first lower thrust gap 52 in the order named. If an air bubble is generated in the lubricating oil 46 during rotation of the shaft 41, for example, the circulation of the lubricating oil 46 causes the air bubble in the lubricating oil 46 to be discharged out of the bearing mechanism 4 through the seal gap 55. Therefore, the radial width of the radial gap 51 is more preferably arranged to be greater than 5 μm and less than about 20 μm.

The pressure of the lubricating oil 46 in the seal gap 55, which is located above and adjacent to the radial gap 51, is equal to atmospheric pressure. Therefore, the pressure of the lubricating oil 46 in an upper end portion of the radial gap 51 is substantially equal to the atmospheric pressure. Since the lubricating oil 46 flows upward in the radial gap 51, the pressure of the lubricating oil 46 in a lower end portion of the radial gap 51 is higher than the pressure of the lubricating oil 46 in the upper end portion of the radial gap 51. Accordingly, the pressure of the lubricating oil 46 in the lower end portion of the radial gap 51 is higher than the atmospheric pressure. In the fan 1, the lubricating oil 46 flows from the radial gap 51 back to the radial gap 51 through the above-sleeve gap 446, the circulation holes 445, and the first lower thrust gap 52 in the order named, and this prevents the pressure of the lubricating oil 46 in the lower end portion of the radial gap 51 from becoming a negative pressure.

In the case of a bearing mechanism in which seal portions are defined in an upper portion and a lower portion of a bearing portion thereof, a sophisticated design is required to prevent a difference in pressure between the seal portions from causing a leakage of the lubricating oil. In contrast, the bearing mechanism 4 of the motor 11 has a so-called full-fill structure, including only one seal portion 55 a, and it is therefore easy to prevent a leakage of the lubricating oil 46 in the case of the bearing mechanism 4. In addition, the surface of the lubricating oil 46 in the seal portion 55 a can be maintained at a substantially fixed position. Moreover, a reduction in the evaporation of the lubricating oil 46 is achieved compared to the case where a plurality of seal portions are provided. In particular, because the seal portion 55 a is arranged in an inner portion of the motor 11, the seal portion 55 a is not exposed to the air currents during the drive of the fan 1. A further reduction in the evaporation of the lubricating oil 46 is thereby achieved. Also, entry of an extraneous material into the seal portion 55 a can be prevented. In the bearing mechanism 4, because the seal portion 55 a is defined around the shaft 41, a leakage of the lubricating oil 46 out of the seal portion 55 a owing to a centrifugal force can be prevented more effectively than in the case where the seal portion is arranged away from and radially outward of the shaft 41.

Because the sum of the axial width of the first lower thrust gap 52 and the axial width of the second lower thrust gap 53 is arranged in the range of about 10 μm to about 40 μm, the fluid dynamic pressures can be generated while ensuring the damper effect owing to the lubricating oil 46. In the bearing mechanism 4, an increase in the radial width of the radial gap 51 leads to a decrease in the pressure of the lubricating oil 46 in the lower end portion of the radial gap 51. However, it is possible to reduce the decrease in the pressure of the lubricating oil 46 in the lower end portion of the radial gap 51 by increasing the sum of the axial width of the first lower thrust gap 52 and the axial width of the second lower thrust gap 53, along with the increase in the radial width of the radial gap 51.

Because the second inclined surface 471 a in which a portion of the first radial dynamic pressure groove array 711 is defined is arranged in the inner circumferential surface 471 of the sleeve 47, the shaft 41 can be sufficiently supported even if the radial gap 51 is widened. Consequently, a reduction in bearing rigidity can be prevented even when the fan 1 is caused to rotate at a high speed or in a high-temperature condition.

Because the motor 11 is a three-phase motor, the motor 11 is capable of being rotated at a high speed. It is therefore easy to cause the frequencies of the vibration that can occur in the motor 11 to deviate from a frequency band that may affect another device in an electronic device in which the fan 1 is installed.

The magnetic member 331 provided in the motor 11 generates the magnetic attraction force that attracts the rotor magnet 22 downward. This contributes to reducing the force that acts to lift the impeller 12 relative to the stationary portion 3 during the drive of the fan 1, and thereby reducing an increase in a bearing loss that occurs in the first lower thrust dynamic pressure bearing portion 691. Moreover, the additional magnetic attraction force that attracts the rotor magnet 22 downward is generated because the magnetic center of the stator 32 is arranged at a level lower than that of the magnetic center of the rotor magnet 22. This contributes to further reducing the increase in the bearing loss that occurs in the first lower thrust dynamic pressure bearing portion 691.

Because the radial dynamic pressure bearing portion 68 is arranged axially between the two balance correction portions 124 a and 125, each of the rotating portion 2 and the impeller 12 is capable of stable rotation, and a further reduction in the vibrations is thereby achieved. In addition, it is possible to reduce the axial length of the radial dynamic pressure bearing portion 68, and to shorten the sleeve 47 of the bearing portion 44. This makes it possible to manufacture the bearing portion 44 with high precision. The axial length of the sleeve 47 is preferably arranged to be less than about four times the outside diameter of the sleeve 47. Because the upper radial dynamic pressure bearing portion 681 is arranged to overlap with the center of gravity of the combination of the motor 11 and the impeller 12 in the radial direction, stability of the rotation of each of the rotating portion 2 and the impeller 12 is increased, and a further reduction in the vibrations is thereby achieved.

While preferred embodiments of the present invention have been described above, it will be understood that the present invention is not limited to the above-described preferred embodiments, and that a variety of modifications are possible.

In a modification of the above-described preferred embodiment, an upper portion of the first radial dynamic pressure groove array 711 may be defined in the second inclined surface 471 a of the sleeve 47 independently of a remaining portion of the first radial dynamic pressure groove array 711. Also, no dynamic pressure grooves may be defined in the second inclined surface 471 a in the bearing portion 44. Even in this case, provision of the second inclined surface 471 a secures an area to support the shaft 41 so that bearing rigidity can be improved to a certain extent.

Each of the first and second radial dynamic pressure groove arrays 711 and 712 may be defined in the outer circumferential surface 411 of the shaft 41. Also, the thrust dynamic pressure groove arrays 721 and 722 may be defined in the upper surface and the lower surface, respectively, of the thrust plate 42. Also, the lower radial dynamic pressure bearing portion 682 may not necessarily be defined in the gap 53 between the lower annular surface 423 of the thrust plate 42 and the upper surface 451 of the thrust cap 45. Also, the circulation holes 445 may be defined as a result of contact of the outer circumferential surface 473 of the sleeve 47 with the inner circumferential surface 483 of the bearing housing 48 with the plurality of grooves defined in the inner circumferential surface 483. Also, the above-sleeve gap 446 may be defined as a result of contact of the upper surface 474 of the sleeve 47 with the lower surface of the annular upper portion 481 with the plurality of grooves defined in the lower surface of the annular upper portion 481.

In a modification of the above-described preferred embodiment, the outer circumferential surface 411 of the shaft 41 may be arranged to include a portion which has a decreased diameter in the vicinity of a top portion of the bearing portion 44 so that the seal portion 55 a may be defined between this portion and the inner circumferential surface 481 a of the annular upper portion 481. A tubular member other than the bearing housing 48 may be arranged above the sleeve 47 in place of the annular upper portion 481. In this case, the seal portion 55 a is defined between an inner circumferential surface of this tubular member and the outer circumferential surface 411 of the shaft 41. In addition, the above-sleeve gap 446 is defined between a lower surface of this tubular member and the upper surface 474 of the sleeve 47. Also, the upper portion of the shaft 41 may be directly fixed to the impeller 12. Also, the shaft 41 may be fixed to the impeller 12 through two or more members. Also, a viscoseal that generates a fluid dynamic pressure through a dynamic pressure groove defined in the seal gap may be used as the seal portion.

A lump of a metal may be arranged, as the weight, in the balance correction portion 125 of the top face portion 123 of the impeller 12. Also, a through hole or a cut portion may be defined as the balance correction portion 125. The same is true of the balance correction portion 124 a of the side wall portion 124. Also, the weight may be arranged on only one of the top face portion 123 and the lower end portion 124 a of the side wall portion 124. Also, the unbalance of the rotating portion 2 may be eliminated by removing a portion of the top face portion 123 or a portion of the side wall portion 124.

In a modification of the above-described preferred embodiment, the magnetic center of the stator 32 and the magnetic center of the rotor magnet 22 may be arranged at the same axial position when the motor 11 is in a stationary state. An additional reduction in the vibrations of the motor 11 can thereby be achieved.

The motor 11 may be used as a motor of a fan of another type, such as a centrifugal fan. A fan in which the motor 11 is used is optimal for use with a device having a hard disk installed therein, such as a server. In the server, the fan is disposed at a position close to the hard disk. Therefore, if the fan is of a type which generates significant vibrations, read or write errors tend to easily occur in the hard disk. In contrast, read or write errors do not easily occur in the hard disk if the fan installed in the server uses the motor 11.

Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises.

The present invention is applicable to fans arranged to produce air currents. 

1. A fan comprising: a motor; and an impeller including a plurality of blades, and arranged to rotate about a central axis through the motor to produce air currents; wherein the motor includes: a stationary portion; and a rotating portion rotatably supported by the stationary portion; the stationary portion includes: a stator; and a bearing portion arranged inside of the stator; the bearing portion includes: a sleeve defined by a metallic sintered body; and a bearing housing arranged to cover an outer circumferential surface of the sleeve; the rotating portion includes: a rotor magnet arranged radially outside the stator; a shaft inserted in the sleeve, and having an upper portion fixed to the impeller directly or through one or more members; and a thrust plate arranged to extend radially outward from a lower end of the shaft to be axially opposed to a lower surface of the sleeve; a radial gap defined between an inner circumferential surface of the sleeve and an outer circumferential surface of the shaft includes a radial dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting axially downward on a lubricating oil, while a thrust gap defined between the lower surface of the sleeve and an upper surface of the thrust plate includes a thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting radially inward on the lubricating oil; a circulation hole extending in an axial direction from an upper surface to the lower surface of the sleeve is defined between the bearing housing and the outer circumferential surface of the sleeve; and the radial gap is arranged to have a radial width in a range of about 5 μm to about 20 μm.
 2. The fan according to claim 1, wherein the radial width of the radial gap is arranged to be greater than 5 μm.
 3. The fan according to claim 1, wherein an outer edge portion of the upper surface of the thrust plate includes an inclined surface arranged to be inclined downward with increasing distance from the central axis.
 4. The fan according to claim 1, wherein the stationary portion further includes a bearing bottom portion arranged axially opposite a lower surface of the thrust plate; and another thrust gap defined between the lower surface of the thrust plate and an upper surface of the bearing bottom portion includes another thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting on the lubricating oil.
 5. The fan according to claim 4, wherein a sum of an axial width of the thrust gap and an axial width of the other thrust gap is arranged in a range of about 10 μm to about 40 μm.
 6. The fan according to claim 1, wherein the bearing housing includes an annular upper portion arranged to extend radially inward on an upper side of the sleeve; and an inner circumferential surface of the annular upper portion and the outer circumferential surface of the shaft are arranged to together define a seal gap therebetween, the seal gap having a radial width gradually increasing with increasing height.
 7. The fan according to claim 6, wherein the inner circumferential surface of the sleeve includes an inclined surface arranged below and adjacent to the seal gap, and having a diameter gradually increasing with increasing height; and the inclined surface is arranged to extend along the outer circumferential surface of the shaft when the shaft is tilted during rotation of the rotating portion.
 8. The fan according to claim 7, wherein the radial dynamic pressure bearing portion includes a dynamic pressure groove, and a portion of the dynamic pressure groove is defined in the inclined surface.
 9. The fan according to claim 1, wherein the radial dynamic pressure bearing portion includes an upper radial dynamic pressure bearing portion defined in an upper portion of the radial gap, and a lower radial dynamic pressure bearing portion defined in a lower portion of the radial gap; and the upper radial dynamic pressure bearing portion is arranged to overlap with a center of gravity of a combination of the motor and the impeller in a radial direction.
 10. A fan comprising: a motor; and an impeller including a plurality of blades, and arranged to rotate about a central axis through the motor to produce air currents; wherein the motor includes: a stationary portion; and a rotating portion rotatably supported by the stationary portion; the stationary portion includes: a stator; and a bearing portion arranged inside of the stator; the bearing portion includes: a sleeve defined by a metallic sintered body; and a bearing housing arranged to cover an outer circumferential surface of the sleeve; the rotating portion includes: a rotor magnet arranged radially outside the stator; a shaft inserted in the sleeve, and having an upper portion fixed to the impeller directly or through one or more members; and a thrust plate arranged to extend radially outward from a lower end of the shaft to be axially opposed to a lower surface of the sleeve; a radial gap defined between an inner circumferential surface of the sleeve and an outer circumferential surface of the shaft includes a radial dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting on a lubricating oil, while a thrust gap defined between the lower surface of the sleeve and an upper surface of the thrust plate includes a thrust dynamic pressure bearing portion arranged to generate a fluid dynamic pressure acting on the lubricating oil; a circulation hole extending in an axial direction from an upper surface to the lower surface of the sleeve is defined between the bearing housing and the outer circumferential surface of the sleeve; and the lubricating oil is arranged to flow from the radial gap back to the radial gap through a gap above the upper surface of the sleeve, the circulation hole, and the thrust gap in an order named during rotation of the rotating portion. 